Shoe press devices conventionally used in a press part for papermaking can be roughly classified into the two types shown in FIGS. 1 and 2.
In both of these types, a roll R and a shoe SH are in surface contact with each other, and two endless felts F1, F2 and a shoe press belt 10A are pinched between the roll R and the shoe SH. The wet paper web P to be dewatered is placed on top of a shoe press belt 10A while being supported between the endless felts F1, F2 and passes the nip press part N composed of the roll R and the shoe SH so as to be dewatered.
In these devices, as shown in FIGS. 1 and 2, a wide nip press part N is made by the surface contact of the roll R and the shoe SH so as to improve the dewatering effect.
In the device of FIG. 1, a relatively long shoe press belt is used; this shoe press belt, which is made into an endless shape, is trained around a plurality of rollers r (5 rollers in FIG. 1) and travels at a fixed tension.
On the other hand, in the device of FIG. 2, a relatively short shoe press belt is used.
FIG. 3 (a) is a cross-sectional view in the cross machine direction (CMD) of the shoe press belt 10A according to the prior art, which can be used in the shoe press devices of FIGS. 1, 2.
This shoe press belt 10A comprises a substrate B, a wet paper web-side layer 20 provided on the outer side of the substrate B and a shoe-side layer S on the inner side; the wet paper web-side layer 20 and the shoe-side layer S are made from a high-polymer elastic material.
A high-polymer elastic material is also provided inside the substrate B. All of the high-polymer elastic materials comprised in the shoe press belt 10A are made into a single body.
The substrate B is provided to give strength to the shoe press belt 10A; substrates made by superimposing a MD (machine direction) yarn and a CMD (cross-machine direction) yarn without weaving, substrates made by winding fine belt-shaped unwoven or woven fabrics into a spiral shape in the widthwise direction, or the like, as well as a base fabric woven from a MD yarn and a CMD yarn, may be used as long as the function as substrate is fulfilled.
In a shoe press belt manufacturing process, the wet paper web-side layer 20 and the shoe-side layer S can be made, in relation to the substrate B, in separate processes or in one process. The high-polymer elastic material may be a gum or an elastomer; among these, however, polyurethane resins, in particular thermosetting polyurethane resins, are frequently used.
A concave water catching part 40, provided in a surface layer 11, which will be explained below, of the wet paper web-side layer 20, has the function of temporarily capturing the moisture squeezed out of the wet paper web at the nip press part N. The moisture captured in the water catching part 40 is thereafter shaken off from the shoe press belt 10A and drained when the shoe press belt 10A travels and its traveling angle changes.
Specifically, the water catching part 40 is made by forming concave grooves continuously provided along the machine direction (MD) or a plurality of blind drill holes independently provided at a depth that does not reach the substrate.
FIG. 3 (a) shows a water catching part 40 in which the cross-section is formed by straight lines and the corners of the bottom part are formed by right angles; however, there are also cases in which the water holding function is fulfilled, wherein the bottom part of the water catching part 40 is entirely curved, as in FIG. 3 (b), the bottom part is a depression with a sharp angle, as in FIG. 3 (c), or wherein the water catching part 40 is in the shape of a so-called dovetail groove with a narrow entrance and a wide inner part, as in FIGS. 3 (d) to (f).
The surface layer 11 of the wet paper web-side layer comprises concave water catching parts 40 and land parts 50, which are the projecting parts produced when the water catching parts 40 are formed.
FIG. 4 is a cross-sectional view in the cross machine direction (the direction intersecting the traveling direction at a right angle) of a conventional shoe press belt wherein the surface layer of the wet paper web side comprising the water catching parts is made from a high-polymer elastic material of high hardness and the other layers are made from a high-polymer elastic material of low hardness. (Patent document 1)
When a shoe press belt is used, extremely strong compression forces working in the thickness direction of the shoe press belt and so-called shearing forces working in the direction opposite the traveling direction repeatedly act on the shoe press belt during the operation of a papermaking machine; therefore, the high-polymer elastic materials gradually deteriorate and, in the end, are unable to follow these loads, with the result that cracks occur from all parts.